The designation "Petzval lend system" is generally applied to a lens system comprising two widely separated lens groups, each of which has a net positive dioptric power, and a negative lens element (or lens group) having a net negative dioptric power. The positive leng groups in combination provide good resolution at high aperture for narrow to moderately wide fields of view. The negative lens element (or lens group) functions as a field flattener. Often, each lens group of a Petzval lens system is a doublet. However, for a particular application, either one (or all) of the lens groups of a Petzval lens system could be a singlet, a triplet or a higher-number multiplet.
The spacing between the two positive lens groups of a Petzval lens system is large in comparison with the thicknesses of the individual lens elements comprising each lens group. Within a lens group, the individual lens elements could be cemented or air-spaced. It is usual for the negative lens element (or lens group) functioning as a field flattener to be positioned immediately in front of the image plane of the Petzval lens system to correct for field curvature. A Petzval lens system that includes a field flattener is often called a flat-field Petzval lens system.
In the prior art, Petzval lens system were typically used only in short-focus applications, e.g., in projectors for 16 mm and 8 mm motion-picture films. Also, Petzval lens systems of the prior art wer typically "achromats", i.e., they were color-corrected at only two wavelengths.